Optical fibres for blown installation

ABSTRACT

An optical fibre package suitable for installation using a fibre blowing process includes at least one optical fibre, an inner coating about said fibre, and an outer resin coating about said inner coating, said resin coating including a significant plurality of particulate inclusions at least 10 μm across, wherein the concentration of the particulate inclusions is significantly greater at the outer surface of the resin coating than at the inner limit of the resin coating. The inclusions may be hollow glass microspheres or mica flakes. The relative absence of significant particulate inclusions at the inner limit of the resin coating improves the mechanical particulate of the fibre package relative to similar known packages. The presence of significant inclusions at the outer surface of the resin coating tends, during a blowing process, to increase viscous drag effects and to reduce friction between the package and the duct into which the package is blown, thereby enhancing blowability.

This is a division of application Ser. No. 08/170,287, filed asPCT/GB92/01190, Jul. 1, 1992 published as WO93/01512, Jan. 21, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to coated optical fibres and methods for theirproduction. In particular the invention relates to coated optical fibreswhich are especially suitable for use in blown fibre systems.

2. Related Art

Optical fibres are now widely used in place of electrical conductors inthe communications field. Typically, glass optical fibres have anexternal diameter in the range 100-150 μm, usually 125 μm. Polymerfibres are normally somewhat larger in diameter. Unlike conventionalelectrical conductors, optical fibres are generally fragile and easilydamaged to the detriment of their performance and lifetime.Consequently, it is important to protect the fibres from damage.

The first step in the protection of optical fibres occurs, at least inthe case of glass fibres, immediately after the fibres are drawn andinvolves the application of one or two layers of synthetic resincoating. This protection, whether one or two layers, is somewhat looselyreferred to as the "primary coating", and fibres so coated are sometimesknown as "primary coated fibres". Alternatively, and more accurately,the coatings are sometimes referred to as the primary and secondarycoatings, and this convention is adopted in this specification. Thefirst coating, which is typically a low-modulus silicone or acrylatepolymer is applied to the fibre surface at a point no more than about ameter from the point where the fibre is drawn down. Commonly, theprimary coating is UV curable. The primary coating is also known as thebuffer layer, since it serves to buffer the fibre from lateral pressure.The reason for applying the primary coating practically as soon as thefibre is formed is that the strength of glass and other small fibresdepends critically on the extent to which their surface is free fromcracks and microcracks. In order to avoid the formation of microcracksit is important to protect the fibre surface from dust and other causesof abrasion, and to this end the zone between the point of fibre drawingand the point of application of the primary coating is kept short anddust-free. The mechanical properties of primary coating materials arecritical to the performance of optical fibres. In particular, thecoating should not induce microbends in the fibre and the mechanicalproperties should be compatible with those of the fibre.

A particularly important consideration is the material's coefficient ofthermal expansion (TCE). The different in TCEs between the material ofthe fibre (normally a silica-based glass, which means a low TCE) andthat of the primary coating (normally with a TCE an order of magnitudeor more greater) means that at low temperatures fibres may be subject toconsiderable compressive stress, significantly increasing optical loss.This effect is generally made worse by increasing the primary coatingthickness, and of course with reduced temperatures.

The secondary coating is typically a hard and robust material, such asnylon, to protect the primary coating, and hence the fibre, from damage.[Increasingly, acrylates, e.g. urethane acrylate, are being used inplace of nylon.] Again, the physical properties of the material are veryimportant in terms of their effect on the optical performance of thefibre, particularly its temperature sensitivity. Particularly now thatoptical fibres are being more widely deployed, it is important thatoptical fibres can be packaged to withstand extremes of temperature. Inpractice, it is insensitivity to low temperatures, e.g. sub-zerocentigrade, which is the most difficult to achieve. For network use incontinental climates, it is desirable that optical fibres should show nosignificant excess loss at temperatures as low as -20°, -40° or even-60° C. Some relevant aspects of the temperature sensitivity of opticalfibres are dealt with in the following papers:

T. A. Lenahan, A. T. & T. Tech. J., V.64, No. 7, 1985, pp 1565-1584, T.Yabuta, N. Yoshizawa and K. Ishihara, Applied Optics, V.22, No. 15,1983, pp. 2356-2362; and Y. Katasuyama, Y. Mitsunaga, Y. Ishida and K.Ishihara, Applied Optics, V. 19, No. 24, 1980, pp 4200-4205.

Conventionally, the primary or secondary coated fibres, which typicallyhave a diameter of about 250 μm, are made up into cables which providesthe required level of mechanical protection for the optical fibres. Itis important to protect the optical fibres from strain, consequently itis usual to decouple the optical fibres from the bulk of the cablestructure. Typically, this decoupling is effected by locating theoptical fibre(s) in a tube or slot in which the fibre is free to move.In addition to decoupling the fibres, it is necessary to ensure that therest of the cable structure can withstand the loads which will beapplied during installation or use of the cable, without imposing excessstrain on its optical fibres. Since the level of strain which opticalfibres can endure without damage is very low, typically less than 0.2percent, cable structures need to be very strong. Typically, opticalfibre cables are installed in much the same way as copper wire cables,that is they are pulled into place through ducts and conduits using arope attached to a cable end. Cables experience very high tensileloadings during such installation, and consequently optical fibre cablesneed very considerable reinforcement to prevent their optical fibresbeing damaged. These requirements increase the size, weight and cost ofoptical fibre cables.

As alternative approach to optical fibre installation is described inour European patent EP-B-0108590. In this method the fibres areinstalled along a previously installed duct using fluid drag of agaseous medium which passes through the duct in the desired direction ofadvance. This method, which is known as Blown Fibre or Fibre Blowinguses distributed viscous drag forces to install a fibre unit which issupported on a cushion of air.

Since the duct is installed first, conveniently using traditional cableinstallation techniques, without any optical fibres and since there isno significant stress imposed on the fibre unit during blowing, it ispossible to use very lightweight fibre structures. Indeed, in terms ofspace-saving and routing flexibility it is desirable if the fibre unitis both small and flexible. Typically, a fibre unit consists of aplurality of conventionally coated optical fibres held together in alightweight polymer sheath which has a foamed coating. Suchmultiple-fibre units may also include a ripcord to facilitate thesplitting out of the fibres from the unit for termination of the fibres.Examples of multiple-fibre units are described in our European patentEP-B-0157610 and EP-A-0296836. Fibre units can also usefully consist ofjust a single fibre provided with a suitably bulky and lightweightsheath, as discussed in EP-B-0157610 and EP-A-029836. An example of asingle-fibre unit is described in EP-A-0338854 and EP-A-0338855.

It has been found to be desirable, e.g. for good blowing performance,for the coatings in a fibre unit to surround the fibres tightly. As aresult of this, the mechanical properties of the fibre unit coatings areas significant to the temperature sensitivity of the optical fibres asthe mechanical properties of the primary and secondary coatings. It isno surprise, therefore, to learn that in EP-A-0296836 the fibre unitcoatings comprises: an inner strength of a material which is soft andhas a low modulus of elasticity, e.g. an acrylate or thermoplasticrubber; an optical intermediate sheath which is hard (greater than 75 DShore hardness) and has a high modulus of elasticity (greater than 900N/nm²), to confer mechanical protection on the soft sheath, and an outersheath of foamed material. This arrangement is akin to the primary andsecondary coatings whose application to individual fibres was describedabove, with the addition of a foamed layer to reduce the fibre unitdensity and hence improve blowability. However, while there is somesimilarity between the requirements made of primary and secondarycoatings and those made of what might be regarded as the tertiary andquaternary coatings, particularly when one is only coating a singlefibre, there are extra constraints which only apply when one is provideda coating system which has to hold several fibres together. Thus, in amultiple-fibre unit one would expect to use materials having largerelastic moduli and in considerably greater thicknesses. Moreover, when amultiple-fibre unit is bent, the individual fibres will generally eachexperience different bending forces and will tend to move relative toeach other. In addition, the larger diameter of multiple-fibre unitsmeans that for a given bend radius the outer surface of the outercoating is exposed to greater tensile and compressive stress than in asingle-fibre unit. It is clear therefore that one cannot necessarilyexpect a coating system which works on a single-fibre unit to work for amulti-fibre unit. A further consideration is that while one might expectstronger coatings to solve the problem of transition from single--tomultiple-fibre units, it has to be borne in mind that the opticalproperties of the optical fibres in a fibre unit are very dependent onthe physical properties of the coatings used in it. In particular, andan mentioned previously, the physical properties of optical fibrecoatings markedly affect the temperature sensitivity of optical fibrescoated therewith. Moreover, the stiffness of a fibre unit markedlyaffects its blowing performance. If a fibre unit is too stiff, it willnot blow--at least in a real-life environment.

Thus, it is by no means clear that a coating system which works for asingle-fibre unit will also work for a multiple fibre unit.

In EP-A-0345968 there is described a range of single-fibre units havingan external coating which comprises a radiation-cured polymer containingparticulate matter. The particulate matter is variously, PTFE particles,hollow glass microspheres, or hollow polymeric microspheres. Theparticulate matter, which preferably has an average particle size ofless than 60 microns, is mixed in with the un-cured liquid polymer. Thefibre to be coated, which may already have a tertiary buffer layer, isdrawn through a bath containing the polymer/particulate mixture to givean outer coating having a thickness in the range 10 to 70 microns. Thecoating is then cured using UV radiation.

We have found that the coating systems as described in EP-A-0345968 arenot suitable for use in sheathing multiple-fibre units. In particular,we have found that such coatings on multiple-fibre units tend to failwhen the unit is bent.

We have found that, particularly with multiple-fibre units such as4-fibre and 8-fibre units, the coating system described in EP-A-0345968for single-fibre units wherein particulate matter is mixed in with theouter coating polymer, produces fibre units which are very prone to"fibre breakout". As a fibre unit is progressively bent, and thusexperiences a progressively smaller bend radius, a certain bend radiusis reached at which irreversible damage to the sheathing occurs allowingthe secondary coated fibres to be exposed. This phenomenon is known asfibre-breakout. If the bend radius at which fibre-breakout occurs (theminimum bend radius) at which fibre unit is likely to experience itsminimum bend radius during normal handling of the fibre unit, the unitis in practice not useable.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved coatingsystem for multiple-fibre units. The invention also seeks to providemulti-fibre units having good long-distance blowing performance and animproved resistance to fibre break-out.

In a fibre aspect the present invention provides an optical fibrepackage for blown installation, the package comprising at least oneoptical fibre and having an outer coating of cured flexible resin, thesurface of said resin coating having been modified prior to the cure ofsaid resin, the effects of the surface modification still beingdetectable. We have found that by modifying the surface of the resinafter its application we can obtain the benefits of increased viscousdrag and/or reduced friction without significantly impairing themechanical properties of the resin. As a consequence it is possible toproduce fibre units of good blowability with good mechanicalproperties--in particular with good fibre break-out performance. Thesebenefits are of particular importance with multiple-fibre units but arealso of value in single-fibre units.

Preferably the surface modification comprises the addition of desiredparticulate matter to the surface of the uncured resin.

The addition of particular matter enables a wide range of surfaceeffects to be achieved and in particular enables the provision of asurface having a much lower coefficient of friction (with respect to aduct wall) than that offered by the resin alone. The material and shapeof the particulate matter can be chosen largely independently andcombinations chosen with a view to achieving a good balance of suchproperties as viscous drag, friction with respect to duct material,break-out resistance, durability, abrasiveness/wear resistance.

Preferably the particulate matter is in the form of balls, for examplemicrospheres. More preferably the balls are hollow.

The round surfaces of balls provide significant surface area to enablegood bonding with the uncured resin surface. The round surface alsomeans that the finished unit is not too abrasive, reducing the wear onthe working surfaces of the blowing equipment and reducing damage to thewalls of ducts during blowing. Additionally, although when very smallballs are used some balls may enter and become completely submerged inthe outer region of the uncured resin surface, the resin surface itselftends to become covered with balls which are stuck like flies onflypaper--that is with the bulk of each ball projecting above the resinsurface. The effect is advantageous in terms of increasing the viscousdrag which is experienced by the unit during blown installation.Additionally, where, as is preferable, the material(s) of the ballshas/have been chosen to be such as to have a low friction coefficientwith respect to the materials which will be used for the duct surface(which will typically be carbon-loaded high density polyethylene but mayfor example be a metal such as stainless steel), glass for example, thefact that the surface of the fibre unit is in effect now provided bysuch a material rather than the resin will mean that the unit has a muchlower co-efficient of friction. Each of these effects alone willcontribute to increasing the blowability (that is the length of the unitwhich can be installed in a given duct under given blowing conditions)of the unit, but the combination of the two effects may synergisticallyincrease blowability.

The use of hollow rather than solid balls enables relatively densematerials, such as glass, to be used without significantly increasingthe mean density of the fibre unit. Indeed the use of hollow balls mayenable the mean density of the fibre unit to be reduced compared to thatfor the fibre unit without particulate additions, even where the ballsconsist of a dense material such as glass. Where materials other thanglass are chosen for the balls they may also be provided in hollow form,with further potential reductions in density.

As an alternative to the use of balls, the particular matter may be inthe form of lumps or flakes.

Preferably a significant plurality of the particulate inclusions are atleast 10 microns across.

While the use of particles less than 10 microns across does not affectbreak-out performance, such small particles tend to offer no improvementin viscous drag effects and there may only be minor reductions infriction. Larger particles are therefore preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic cross-sectional view through a two-fibre packageaccording to the invention;

FIG. 2 is a schematic cross-sectional view through a four-fibre packageaccording to the invention;

FIG. 3 is a schematic cross-sectional view through a eight-fibre packageaccording to the invention;

FIGS. 4 and 4A show schematically the apparatus and test used toascertain fibre break-out radii;

FIGS. 5 and 5A show a prior art fibre unit which has been subject tobreakout.

FIGS. 6 and 6A show a photomicrograph and analysis of an end section ofan 8-fibre unit according to the invention;

FIGS. 7 and 8 are plots representing fibre package blowability;

FIG. 9 is a plot showing the effects on fibre attenuation of temperaturecycling on a four fibre package at different wavelengths;

FIG. 10 is a plot of a friction analysis of several fibre unit types;

FIG. 11 is a schematic illustrating the method used to assess thefrictional behaviour of the units analysed in FIG. 10;

FIG. 12 is a plot of installation force for several fibre unit types;

FIG. 13 is a schematic illustrating the major elements in a productionline for producing fibre unit;

FIG. 14 shows processing details for the production of a typical fibreunit according to the invention;

FIG. 15 is a sectional diagram showing details of a coating chamber forsurface modification according to the invention; and

FIGS. 16 to 18 are optical micrographs of four-fibre packages accordingto the invention;

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a two-fibre package having an outer resin layer withmodified surface. The two conventional fibres 1, which have both primaryand secondary coatings, are nominally 260 μm in diameter. The fibres arepositioned on either side of the centre line of the package, so thattheir secondary coatings just touch. The fibres are held in a softbuffer layer 2 which has an overall diameter of about 760 μm. In thisexample the buffer layer comprises a silicone-acrylate, Cablelite950-701 (available from DSM Desotech, The Netherlands). About thisbuffer layer there is a further resin layer 3 which is a tough layerwhich serves to protect the buffer layers and fibres from mechanical andchemical attack. In this example layer 3 comprises Cablelite 950-705, aurethane-acrylate resin, and is about 50 μm thick. It is this layerwhose surface is modified. In the present example, the modificationconsists of the inclusion and addition of glass microspheres 4 to theresin surface. The microspheres which are attached to the resin surfaceonly after the application of the outer resin layer 3 to the curedbuffer layer 2, are in this example hollow glass microspheres sold underthe tradename "Q-CEL 500" by the PQ corporation (PO Box 840, ValleyForge, Pa. 19482, U.S.A.). The mean size (effectively the O.D.) of themicrospheres is 68 μm (with a range of 10-180 μm). The microspheres areapplied to the resin surface in such a way that they do not penetrate asfar as the interface between the resin layer 3 and the buffer layer 2.The resin layer 3 is cured after the microspheres have adhered to it,leaving the microspheres trapped like flies on flypaper. The presence ofthe hard microspheres greatly reduces the friction which exists betweenthe fibre package and the wall of the duct into which the package isblown. The reduction in friction contributes to the enhanced blowabilityof this package. A second factor which improves blowability is theenhanced viscous drag which the rough surface provides during blowing.

A similar 4-fibre package is shown in FIG. 2. Here the four fibres 1 aredisposed symmetrically about and equidistant from the axis of thepackage. Again the secondary-coated fibres are arranged so that theirsecondary coatings just touch. Inter alia this helps to reduce theincidence of microbending due to thermal contraction/expansion.

Three examples of 4-fibre package units, numbered 347, 348 and 349, weremade. In all these examples the buffer layer 2 comprised Cablelite3287-9-39 and the outer layer 3 comprised Cablelite 950-705, both beingurethane-acrylate resins available from DSM Desotech, the Netherlands.The '39 resin had a secant modulus at 2.5% strain of about 1.0 MPa, atensile strength of 1.3 MPa, a Shore D hardness of 49 and a 115%elongation. The 705 resin has a tensile modulus at 2.5% strain of about700 MPa, and a 43% elongation. The microspheres in these 4-fibreexamples were hollow glass microspheres sold under the tradename "Q-CEL520 FPS" by the PQ corporation. The mean size (effectively the O.D.) ofthese microspheres was 35 μm (with a range of 25-45 μm). Again themicrospheres were applied to the resin surface in such a way that theydid not penetrate as far as the interface between the resin layer 3 andthe buffer layer 2. The resin layer 3, was cured after the microsphereshad adhered to it.

The sizes for the three examplar 4-fibre Units were as follows:

    ______________________________________    Unit             347        348    349    Diameter of layer 2 (μm)                     789        788    788    Diameter of layer 3 (μm)                     930        924    913    Thickness of layer 3 (μm)                     70.5       68     62.5    Weight (gm.sup.-1)                     0.69       0.70   0.71    ______________________________________

FIG. 3 shows a similar 8-fibre unit. Four of the eight fibres arearranged as in the previous 4-fibre units. The other four fibres, theouter four, are again symmetrically disposed about the axis of thepackage, this time on the centre lines which separate adjacent ones ofthe inner four. As before, the fibres are disposed so that theirsecondary coating just touch. The overall diameter of the buffer layerin this case is about 1 mm and that of the outer layer is about 1.2 mm.

In FIG. 6 can be seen a micrograph of a section through an 8-fibre unitaccording to the invention. The micrograph, which shows some debris andresin loss resulting from the sectioning procedure, shows the extent ofmicrosphere penetration into the outer resin layer. This is more clearlyseen from the analysis (FIG. 6B) which accompanies FIG. 6A where theextent of penetration has been somewhat exaggerated as it includes theeffect of debris.

We have carried out comparative tests to determine the bend radius atwhich fibre breakout will occur in a fibre unit sheathed according tothe technique of EP-A-0345968 and a fibre unit wherein only the surfaceof the outer layer has been modified, that is according to the presentinvention.

The apparatus used is shown in FIG. 4A (and the exploded view at FIG.4A). Two grooved plates (5 and 6) are mounted parallel to each other onguide rods (7). A motor (8) is mounted on one guide rod arranged so asto allow the motor to drive one grooved plate (6) towards the outerwhile maintaining the two plates in parallel. In use the fibre unitunder test is placed in the groove in each plate and allowed to adopt afree loop shape between the plates. The distance D between the plates isthen slowly reduced and the apex of the fibre unit loop is carefullyobserved for signs of fibre breakout.

The fibre unit radius at which this is first observed is the minimumbend radius.

The results of these comparative tests are shown in Table 1.

                  TABLE 1    ______________________________________    minimum bend radii                         Microspheres applied to    Microspheres mixed in                         surface of outer layer    outer layer resin    resin (according to the    (as in EP 345968)    present invention)    ______________________________________    4-fibre 10 mm ± 2     2 mm      ± 0.5    unit    8-fibre 50 mm ± 5     25 mm     ± 2    unit    ______________________________________

The microspheres, outer layer resins and unit diameters used in thesetests were identical for each of the two techniques for applyingmicrospheres. As can be seen from table 1 the fibre units produced bythe method of EP-A-0345 968 have significantly larger minimum bend radiithan those produced by modifying only the surface of the outer layerresin. In practice we have found that when handling fibre units withmicrospheres mixed in to the outer layer resin fibre breakout was asignificant problem. Indeed we found such fibre units unusable for allpractical purposes. From a comparison of our new product and those madeaccording to EP 345968 we have concluded that this poor breakoutperformance is probably due to the presence in the EP345968 products ofmicrospheres at the interface between the outer and inner resin layers.

FIG. 5A is a photomicrograph showing an example of a fibre unitaccording to EP345968 which has suffered fibre-breakout; theaccompanying analysis (FIG. 5A) shows more clearly the nature of thefailure.

In a further comparative test two 4-fibre units, one coated according tothe present invention and one coated according to the technique ofEP-A-0345968 were wrapped loosely around a mandrel of 40 mm diameter andplaced in an oven at 60° C. After 100 hours the unit coated according to'968 had suffered fibre breakout, whereas even after 1000 hours the unitcoated according to the present invention had not suffered fibrebreakout.

A further factor which we have found to be important in preventing fibrebreakout is the choice of an outer layer resin with an appropriatepercentage elongation figure. Manufacturers such as DSM Desotech measurethe percentage elongation of a resin by stretching a cast film of theresin to its elastic limit.

A film of the resin of thickness 70 to 80 microns is first formed bycuring with a dual D end mercury lamp which provides 3.5 Jcm⁻² of U.V.radiation at the film surface. Then the film is stretched in anenvironment of 22° to 24° C. and 50 to 55% relative humidity and thepercentage elongation at failure is recorded.

We have found in practice that for a four fibre unit produced byapplying microspheres to the surface of the outer layer an appropriatepercentage elongation for the outer layer resin is approximately 35%.One such suitable resin is cablelite 950-705. We have found that a resinwith a percentage elongation of approximately 15% (such as Cablelite3287-9-31) is not sufficiently flexible to avoid fibre breakout whenused for the outer layer of a 4-fibre unit.

For an 8-fibre unit we have found that due to its large diameter ahigher percentage elongation is required of the outer layer resin thanfor a 4-fibre unit.

A method which we have found useful for estimating the percentageelongation required for different unit sizes is to scale the percentageelongation in accordance with the longitudinal extension experienced bythe outside of a loop of a unit. For example a four fibre unit of 0.93mm outer diameter formed into a loop of radius 10 mm would have alongitudinal extension at the outside of the loop of ##EQU1##

While an 8-fibre unit of 1.3 mm outer diameter bent into a loop of thesame radius would have a longitudinal extension of ##EQU2##

Since we have found in practise that a material of approximately 35%percentage elongation is satisfactory for such a 4-fibre unit we canestimate that a percentage elongation of ##EQU3## should be satisfactoryfor an 8-fibre unit.

Initial tests carried out with two high percentage elongation resinsfrom DSM Desotech indicate that, in fact, a percentage elongation ofapproximately 40 (as measured by the technique described above) may besufficient to avoid fibre breakout in an 8-fibre unit at a radius of 10mm. The two high elongation resins were RCX-4-207 and RCX-4-208 havingpercentage elongations of 42 and 40 respectively.

In increasing the percentage elongation of a resin for use in the outerlayer of a high fibre count unit a consequent increase in the frictionalproperties of the resin should be avoided as far as this is possible.

Some plots of blowability tests for fibre packages of the type shown inFIGS. 2 and 3 are shown in FIGS. 7 and 8. Note that FIG. 7 is forblowing into a trial duct network of 3.5 mm bore duct spread over a 4acre site, the duct length being just over a kilometer. FIG. 8 is for atest carried out using a 300 meter duct wound in 4 layers around a 0.5meter diameter drum.

FIG. 9 shows the effects on fibre attenuation of temperature cycling asample 4-fibre unit.

To achieve good blowability of a fibre package unit low friction betweenthe unit and the duct into which it is being installed is required. FIG.10 shows a comparison between the coefficient of friction of two unitsmodified with microspheres, a unit modified with mica flakes and anunmodified unit. The coefficient of friction is measured by attaching aweight (shown along the x-axis of FIG. 10) to one end of a unit whichhas been would around an 85 mm diameter glass tube and applying a knownforce to the other end of the unit. Referring to FIG. 11 the coefficientof friction is calculated from μ=(lnT₁ -lnT₂)/2πN. The average of μ forfive traversing speeds (controlled by T₂) for each weight T₁ is taken.From FIG. 10 it can be seen that the coefficient of friction for boththe microsphere modified units is lower than for the other two units.

A further factor which affects blowability is any enhanced viscous dragprovided by modification of the surface of a unit. This factor can beassessed by measuring the installation force generated on a unit whileit is being installed into a short length of duct (a short length isused so that contributions from frictional forces are negligible).

FIG. 12 shows this installation force measured for various units. SF12microspheres have a mean size of 65 μm, while CPO3 microspheres have amean size of 10 μm.

As can be seen all the modified units have improved viscous drag overthe unmodified unit.

The fibre packages shown in FIGS. 1 to 3 were all made on what isessentially a standard multifibre packaging line of the type used forthe manufacture of ribbon cables. Suitable equipment can be obtainedfrom Heathway Limited of Milton Keynes.

The equipment modifications which were needed for the manufacture of thepackages shown in FIGS. 1 to 3 are limited to the coating dies plus theaddition of surface modification equipment.

Briefly described with reference to FIG. 13, the processing sequence isas follows. The fibres are used straight from the drums on which theyare dispatched by manufacturers. The drums are mounted on one or morepay-off stands from which they are fed under tension, via individual orcommon guide wheels, to the first pressurised coating system 17. Thefibres 27 pass down the tower, through the first pressurised coatingsystem 17, through a resin-curing system 18 (typically including a UVlamp system), possibly through a size monitor, through a secondpressurised coating system 14, through a surface modification zone 26(e.g., with fluidized microspheres 25 being electrostatically appliedpursuant to control unit 24), through a resin-curing system 23 (againtypically including a UV lamp system), possibly through anothermeasuring or inspection unit, and then on, via a capstan, to a drum orpan winding system 22. Typically the winding system will be not part ofthe processing tower.

FIG. 14 shows the parameters used in the above process to produce unit348 and these parameters are typically of those used to produce all 4fibre units,

The coating die which is used in each case to produce the inner coatedstructure, that is the layer 2 in FIGS. 1 to 3, is of specialsignificance for all multiple fibre counts. This coating die is locatedin the first pressurised coating system. This inner coating die isspecially profiled to hold the fibres in registration prior to theapplication of the resin 2. This enables an accurately centred structureto be produced, facilitating the achievement of uniform coatingthicknesses and hence improved performance and stability. The outercoating die, which is used for the application of the outer resin layer,is a standard fibre coating profile.

The die arrangement for the pressurized coating system is as follows. Adie body has a fibre input, a fibre output and a pressurized resin feed.There is a first die on the fibre input and a second, larger die on thefibre output. The size and shape of the output die determine the sizeand shape of coating achieved. In the present embodiments we are onlyinterested in achieving resin coatings of circular cross-section, so theoutput die is shaped accordingly. On the second pressurised coatingsystem the incoming fibre assembly already has the first resin coat, thebuffer layer 2 of FIGS. 1 to 3. Thus a circular orifice, appropriatelylarger than the buffer layer diameter, is suitable for the inlet-end dieon the second coating system.

It is the inlet-end die of the first pressurised coating system which isused to ensure the registration of the fibres at the time of coatingwith the first resin layer. In the case of the 2-fibre unit, the diethroat is elliptical with the major axis of the ellipse approximatelytwice the minor axis, and the minor axis being about 10% larger than thediameter of the optical fibres which are to be packaged. The die inletis of circular section and, as is conventional, flared. To avoid wearand damage to the fibre and dies, in each die head the two dies shouldbe concentric and accurately aligned with the fibre path and each other.

For the 4-fibre package the die throat of the relevant die is similar tothat for the 2-fibre package except that the "ellipse" has major andminor cases of virtually the same length. In practice, the long axis isas in the 2-fibre package case and the "short" axis is double the lengthof the short axis used in the 2-fibre package case.

For the 8-fibre package the die throat is a more complicated shape,reflecting the desired disposition of fibres in the package. Again, a10% clearance is provided relative to the nominal outline of the fibresin the package.

In view of the relatively tight tolerance that is required of the diedimensions if the fibre package is to be suitably uniform, it is ofcourse important to ensure that the incoming fibres are supplied to asuitably tight specification.

In this embodiments shown in FIGS. 1 to 3, the surface modification wasachieved by adhering glass microspheres to the surface of the resin 3.The microspheres come as a free-flowing powder. Because of their smallsize and low density, it is readily possible to fluidise or aerate amass of them. By causing there to be such aerated mass about the wetresin surface 3 between the second pressurised coating system and thesecond resin curing location, it is possible to get microspheres toadhere to the wet resin. To assist this process and to give a uniformdistribution of microspheres on the surface of the package, it ispreferable to electrostatically charge the microspheres after they havebeen fluidised. This is done by passing the fluidised microspheresthrough a conventional electrostatic spray gun operating at 10-100 kVbefore they are directed at the fibre package. The charged microspheresare attracted to the fibre package while being mutually repulsive, thusfacilitating a controlled, uniform coverage of the fibre packagesurface.

FIG. 15 shows an enlarged view in cross-section of the coating chamber26 of FIG. 13, in which the electrostatic gun 32 with its input offluidized particles 31 (and vacuum extraction of excess particles at 29)can be seen. The coated fibre 28 (with uncured top coat) has its surfacemodified upon passage therethrough to become a modified surface fibre 20before passage to curing station 23.

In addition to the above-mentioned Q-CEL 500 and Q-CEL 520 FPSmicrospheres, there are other suitable microspheres. Q-CEL 400(available from AKZO Chemicals), which has a mean particle size of 75 μm(80% in the range 10-120 μm) and a lower density than Q-CEL 500, Q-CELSF, which has a 100% size range of 10-125 μm, with a mean of 65 μm,"Extendospheres XOL 70", with a nominal size of 70 μm, and"Extendospheres SF12" with a mean size of 65 μm are particularlysuitable.

FIGS. 16 and 17 are optical micrographs of four-fibre units. Both unitshave a buffer layer of Cablelite 950-701 and an outer layer of Cablelite950-705. The microspheres (Extendospheres SF12) were appliedelectrostatically using a gun voltage of 90 KV and can be seen to beuniformly distributed on the outside surface of the outer layer. Theunits shown in FIG. 18 have Q-CEL 500 microspheres.

As an alternative to the use of microspheres we have achieved goodresults with thin platelets of natural mica, with sizes in the range40-200 μm, which are sold as pigments under the tradename "Iriodin" byMerck. Again, we have found that the use of electrostatic coating isadvantageous.

More irregularly shaped non-plate-like particles, i.e. lumps, may becourse be used in place of the microspheres or platelets. The lumps maybe of glass or of a polymer which will give rise to low friction of thefibre unit against the relevant duct material, for example PTFEparticles such as those described in EP-A-0345968 could be used.

It should be noted of course that the microspheres are not, and need notbe, perfectly spherical.

A further alternative is to modify the resin surface without theaddition of particulate matter. By passing the uncured second resin 3through a ring of air jets directed at the coating surface we haveachieved a significant and useful texturing. Again of course the resinis cured after being textured.

The following tests have been used to demonstrate the mechanicalperformance of the fibre packages according to the invention. Nopermanent damage is defined as:

(a) the reversibility of attenuation with ±0.05 dB measured at awavelength of 1300 nm at 20° C.

(b) a maximum change in diameter ratio of 0.05, after any mechanicaltest. Where diameter ratio is the ratio of the maximum diameter to theminimum diameter of the fibre unit.

Strength

The fibre unit shall have sufficient strength to withstand a tensileload equivalent to the weight per unit length of the fibre packagemultiplied by the maximum blowing length. The load shall not produce atotal strain exceeding 0.25% in the fibres, and shall not causepermanent damage to the component parts of the unit. The load shall besustained for 10 minutes and the strain of the fibres monitored. Thetarget value for the residual strain in the fibres after the load isremoved is zero. A maximum of 0.05% would be acceptable for productionunits.

Flexibility

The fibre and the component parts of the package shall not sufferpermanent damage and shall regain circularity when the package isrepeatedly wrapped and unwrapped by hand 4 complete turns for 10complete cycles, around a mandrel 40 mm in diameter.

Compressive Stress

The fibres and the component parts of the package shall not sufferpermanent damage during the application of a compressive load of 50Newtons applied between two flat plates of dimensions 50 mm by 50 mm.The plate edges shall have a radius of 3 mm. The load shall be appliedfor a period of 60 seconds.

Also a destructive compressive test is applied in which a compressiveload of 500 Newtons is applied for a period of 15 minutes. As a resultof this test the fibre must not be broken, but the fibre unit need notmeet the diameter change test described above.

Blowability

Type 1 Tube

The fibre unit shall be blown into a 300±30 m length of BT-approved 3.5mm ID tubing wound onto reels of 500 mm barrel diameter and 250 to 300mm between flanges.

The installation period for this length shall not exceed 30 minutesusing standard BT approved blowing apparatus.

Type 2 Tube

The fibre unit shall be blown into a 1000±30 m length of BT approved 3.5mm ID tubing installed into a BT-approved route.

The installation period for this length shall not exceed 100 minutesusing standard BT approved blowing apparatus.

These blowing tests apply over the temperature range 0° to +60° C.

Fibre units 347, 348 and 349, blew into a 300 meter length of 3.5 mmbore duct, wound about a drum 0.5 meter diameter, in between 12 minutesand 12 minutes 40 seconds, with an applied pressure of 7 bar. Theblowing head was a wheeled head as described in EP-B-108590 and the ductwas as described in EP-A-432171. The average installation rate wastherefore about 24 meters per minute.

It is believed that the blowability of units according to the presentinvention is improved, at least when hard microballs are used, becausethe microballs are not coated in the resin of the fibre unit, unlikethose made according to the technique of EP-A-345968.

Of course units made according to the present invention may comprisesingle-mode fibres or multimode fibres or both together.

We claim:
 1. An optical fibre package for blow installationcomprising:at least one optical fibre, and an outer layer of curedflexible resin, the surface of said resin layer having been modifiedafter the formation of the layer but prior to the cure of said resin,the resin layer including an inner zone, remote from the surface, whichis unaffected by the surface modification process, effects of thesurface modification still being detectable.
 2. A package as in claim 1containing at least two optical fibres, wherein the package, at 20° C.,has a minimum bend radius at which fibre breakout occurs of less than 10mm.
 3. A package as in claim 2, wherein the package comprises fouroptical fibres.
 4. A package as in claim 3, having only four opticalfibres wherein the outer layer of resin has a percentage elongationgreater than 15 at 20° C.
 5. A package as in claim 1, wherein said outerlayer of resin has a percentage elongation greater than 30 at 20 degreescelsius.
 6. A package as in claim 1 wherein the package comprises eightoptical fibres and said outer layer of resin has a percentage elongationgreater than 35% at 20° C.
 7. Use of an optical fibre package as inclaims 1, 2, 3, 5, 6 or 4 in a blown fibre installation process whereinthe optical fibre package is installed along a previously installed ductusing fluid drag of a gaseous medium which passes through the duct inthe desired direction of advance.
 8. An optical fibre package for blowinstallation comprising:at least one optical fibre, and an outer layerof cured flexible resin which has been non-homogeneously modified afterformation of the layer, but prior to cure of the resin, to leave adetectable surface modification without substantially modifying an innerzone of the layer extending outwardly from said at least one opticalfibre.
 9. An optical fibre package for blow installation comprising:atleast one optical fibre, and an outer layer of cured flexible resinextending radially outwardly from said at least one fibre, said resinhaving been subjected just prior to curing to modifications whichincrease as the radius increases to leave a detectable surfacemodification after the resin is cured without substantially modifying aradially innermost zone of the layer.